To JavaScript, the disabled attribute above is recognized as
simply the disabled property of that particular element. Being a property,
we should all know how to access it, then. Ok, you forgot. Here goes:

document.formname.elementname.disabled

So, to access the above disabled attribute, I would first slap on a name
for both the <form> and <input> tag, and take it from there:

The disabled property of JavaScript is a Boolean property, meaning it
only take two possible values: "true", or "false". By knowing this, you
basically know how to manipulate the disabled attribute- disabling and
re-enabling a form element at will. Below demonstrates an example where
clicking a link disables/ enables a form button: